Updating for Yadda 0.6.2

Thanks for the webdriver example. Really useful. I've updated for Yadda 0.6.2 (significant API change to the mocha plugin, triggered by [Issue 23](https://github.com/acuminous/yadda/issues/23)) and also tweaked it wrt to the shared context.

One thing that's worth noting, it's absolutely fine to load all the step definitions / libraries in advance, however once you're a few months into development, you might find the steps clashing. You can get around this by importing a subset of step definitions / libraries per feature.